The present invention relates to an apparatus for making ac surface photovoltage measurements of a specimen of semiconductor material under dc bias voltage conditions.
Known patents relating to making surface photovoltage (SPV) measurements of semiconductors include U.S. Pat. No. 4,544,887 which issued on Oct. 1, 1985 in the name of E. Kamieniecki and which discloses a method of measuring the photo-induced voltage at the surface of a specimen of semiconductor material; U.S. Pat. No. 4,286,215 which issued on Aug. 25, 1981 in the name of G. L. Miller and which discloses a method and apparatus for the contactless monitoring of the carrier lifetime in semiconductor materials; U.S. Pat. No. 4,333,051 which issued on June 1, 1982 in the name of A. M. Goodman and which discloses a method and apparatus for determining minority carrier diffusion length in semiconductors; U.S. Pat. No. 4,433,288 which issued on Feb. 21, 1984 in the name of A. R. Moore and which discloses a method and apparatus for determining minority carrier diffusion length in semiconductors; and U.S. Pat. No. 4,663,526 which issued on May 5, 1987 in the name of E. Kamieniecki and which discloses a method and apparatus for the nondestructive readout of a latent electrostatic image formed on an insulating material.
Known publications relating to making surface photovoltage measurements of semiconductors include Emil Kamieniecki, "Surface Photovoltage Measured Capacitance: Application To Semiconductor/Electrolyte System", J. Appl. Phys. Vol. 54, No. 11, November, 1983, pp. 6481-6487; Emil Kamieniecki, "Determination of surface space charge capacitance using a light probe", J. Vac. Sci. Technol., Vol. 20, No. 3, March, 1982, pp. 811-814; Hiromichi Shimizu, Kanji Kinameri, Noriaki Honma and Chusuke Munakata, "Determination of Surface Charge and Interface Trap Densities in Naturally Oxidized n-Type Si Wafers Using ac Surface Photovoltages", Japanese Journal of Applied Physics, Vol. 26, No. 2, February, 1987, pp. 226-230; A. Sher, Y. H. Tsuo, John A. Moriarty, W. E. Miller and R. K. Crouch, "Si and GaAs Photocapacitive MIS Infrared Detectors", J. Appl. Phys., Vol. 51, No. 4, April 1980, pp. 2137-2148; Olof Engstrom and Annelle Carlsson, "Scanned Light Pulse Technique For the Investigation of Insulator-semiconductor Interfaces", J. Appl. Phys. Vol. 54, No. 9, September, 1983, pp. 5245-5251; E. Thorngren and O. Engstrom, "An Apparatus for the Determination of Ion Drift in MIS Structures", J. Phys. E: Sci, Instrum., Vol. 17, 1984, printed in Great Britain, pp. 1114-1116; E. Kamieniecki and G. Parsons, "Characterization of Semiconductor-Electrolyte System by Surface Photovoltage Measured Capacitance", 164th meeting of the Electrochemical Society, Washington, D.C. October, 1983; R. R. Chang, D. L. Lile and R. Gann, "Remote Gate Capacitance-Voltage Studies for Noninvasive Surface Characterization", Appl. Phys. Lett. Vol. 51, No. 13, Sept. 28, 1987, pp. 987-989; Chusuke Munakata, Shigeru Nishimatsu, Noriaki Honma and Kunihiro Yagi, "AC Surface Photovoltages in Strongly-Inverted Oxidized p-Type Silicon Wafers", Japanese Journal of Applied Physics, Vol 23, No. 11, November 1984, pp. 1451-1461; R. S. Nakhmanson, "Frequency Dependence of the Photo-EMF of Strongly Inverted Ge and Si MIS Structures - I. Theory", Solid State Electronics, 1975, Vol 18, pp. 617-626, Pergamon Press, Printed in Great Britain; R. L. Streever, J. J. Winter and F. Rothwarf, "Photovoltage Characterization of MOS Capacitors", Proc. Int. Symp. Silicon Materials Sci & Tech., Philadelphia, May 1977 (Electrochem. Soc. Princeton, 1977) pp. 393-400; R. S. Nakhmanson, Z. Sh. Ovsyuk and L. K. Popov, "Frequency Dependence of Photo-EMF of Strongly Inverted Ge and Si MIS Structures - II Experiments", Solid State Electronics, 1975, Vol. 18, pp. 627-634 Pergamon Press, Printed in Great Britain; Chusuke Munakata and Shigeru Nishimatsu, "Analysis of ac Surface Photovoltages in a Depleted Oxidized p-Type Silicon Wafer", Japanese Journal of applied Physics, Vol 25, No. 6, June, 1966, pp. 807- 812; Chusuke Munakata, Mitsuo Nanba and Sunao Matsubara, "Non-Destructive Method of Observing Inhomogeneities in p-n Junctions with a Chopped Photon Beam", Japanese Journal of Applied Physics, Vol. 20, No. 2, February, 1981, pp. L137-L140; Chusuke Munakata and Shigeru Nishimatsu, "Analysis of ac Surface Photovoltages in a Depleted Oxidized p-Type Silicon Wafer", Japanese Journal of Applied Physics, Vol 25, No. 6, June, 1986, pp. 807-812; S. M. Sze, "MIS Diode and Charge-Coupled Device", Physics of Semiconductor Devices, John Wiley & sons Inc. New York 1981, second edition, pp. 362-394.
In copending U.S. patent application Ser. No. 145,943 filed on Jan. 20, 1988 in the name of E. Kamieniecki and assigned to the assignee of this application there is disclosed a method and apparatus for the characterization of a specimen of semiconductor material which involves making ac surface photovoltage measurements under bias voltage conditions. The apparatus includes a reference electrode, a back electrode, an insulator, a light source whose output is intensity modulated and an adjustable dc bias voltage source. The reference electrode is a rigid structure in the form of conductively coated glass plate having an area at least as large as the specimen of semiconductor material. The back electrode is a sheet of conductive material, such as aluminum foil, and is fixed to a rubber pad which is attached to a mechanical press. The insulator is a free standing element in the form of a sheet of transparent flexible dielectric material, such as mylar. The insulator and the back electrode are at least equal in size (area) to the semiconductor. In use, the specimen is positioned between the back electrode and the reference electrode, with the reference electrode being electrically separated from the specimen by the insulator so as to provide a capacitance type of pickup. The modulated light beam from the light source is focussed through the reference electrode and insulator to a spot on the semiconductor surface and its location on the semiconductor surface adjusted by a scanning mechanism. When the SPV measurements are being taken, the semiconductor is pressed up against the insulator by pressure applied by the mechanical press through the rubber pad, the pressure being sufficient to insure close compliance between the specimen and the reference electrode. As the bias voltage, which is applied between the back electrode and the reference electrode, is varied over a wide range (i.e. from about 0 volts up to about 500 volts), the ac surface photovoltage signals which are stimulated by the intensity modulated beam of light are picked up between the back electrode and the reference electrode and fed into the electronics part of the apparatus for processing.
According to the method described in the above noted pending patent application for characterizing properties of the specimen being examined, the surface photovoltage is measured under certain defined conditions. The defined conditions of measurement are as follows: (1) the wavelength of the illuminating light is shorter than that corresponding to the energy gap of the semiconductor material, and (2) the light is intensity modulated with the intensity of the light and the frequency of modulation being selected such that the induced (ac) voltage signal is directly proportional to the intensity of light and reciprocally proportional to the frequency of modulation. When measured under these conditions, the surface photovoltage is proportional to the reciprocal of the semiconductor space charge capacitance. If the surface of the specimen is illuminated uniformly this relationship may be expressed as ##EQU1## where .delta.V.sub.s is the surface photovoltage, C.sub.sc is the space charge capacitance. .phi. is the incident photon flux, R is the reflection coefficient of the semiconductor material, f is the modulation frequency of the light, and q is the electron charge. K is equal to 4 for squarewave modulation of light intensity and is equal to 2.pi. for sinusoidal modulation. If the illumination of the semiconductor surface is local and not uniform, .delta.V.sub.s is first determined by using the equation .delta.V.sub.m =(s/s).delta.V.sub.s where V.sub.m is the measured voltage, s is the area of the illuminated portion (plus diffusion) and S is the total area of the semiconductor. The semiconductor space charge capacitance C.sub.sc is then determined using equation (1) noted above.
Apparatus as described in the above noted pending patent application has been constructed in the laboratory. Recurrent problems with the apparatus as constructed have been caused by the conflicting needs to (i) apply sufficient pressure to hold the insulator motionless during the sweep of the bias field voltage and maintain the reference electrode in close compliance (i.e. locally parallel relationship) with the semiconductor and (ii) avoid high voltage breakdown of the insulator which may become damaged by the physical holding forces and dust particles and/or breakage or contamination of the semiconductor itself. Insulator motion during the bias sweep is caused by the electrostatic forces produced by the bias field. These forces may cause the insulator to move, changing the distance between the reference electrode and the semiconductor during the bias sweep. Changes in the distance between the reference electrode and the semiconductor will cause local variations in the coupling capacitance (i.e. the capacitance between the reference electrode and the specimen) and therefore disturb output photovoltage signal. Such changes may also cause modulation of the optical interference fringes in the narrow gaps between the insulator, the specimen and reference electrode. This light intensity modulation can distort the results and render it uninterpretable.
As can be seen, there is a need for a new arrangement for detecting ac surface photovoltage signals in a specimen of semiconductor material, especially under high bias voltage conditions.
Accordingly, it is an object of this invention to provide a new and improved apparatus for making ac surface photovoltage measurements of a specimen of semiconductor material;
It is another object of this invention to provide a new and improved apparatus for making ac surface photovoltage measurements of a specimen of semiconductor material under dc bias voltage conditions.
It is still a further object of this invention to provide a new and improved capacitive type reference electrode for use in making ac surface photovoltage measurements of a specimen of semiconductor material.
It is yet a further object of this invention to provide a new and novel reference electrode assembly which is especially constructed for use in an apparatus for making ac surface photovoltage measurements of a specimen of semiconductor material under high dc bias voltage conditions.
The present invention overcomes the difficulties noted above with the apparatus described in the above noted patent application.